This page describes the various init systems which are available as alternatives to systemd
For a more comprehensive explanation of init, and runlevels, see wikipedia.org Page: Init
Init is the first process started during system boot. It is a a daemon process that continues running until the system is shut down. Init is the direct or indirect ancestor of all other processes, and automatically adopts all orphaned processes. It is started by the kernel using a hard-coded filename; if the kernel is unable to start it, panic will result. Init is typically assigned process identifier 1.
The init scripts (or rc) are launched by the init process to guarantee basic functionality on system start and shutdown. This includes (un)mounting of file systems and launching of daemons. A service manager takes this one step further by providing active control over launched processes, or process supervision. An example is to monitor for crashes and restart processes accordingly.
These components combine to the init system. Some inits include the service manager in the init process, or have init scripts in close relation to them. These inits are below referred to as integrated, though entries in different categories may explicitly depend on each other.
A nice (but still non-comprehensive) overview of init systems can be found in this blog entry, titled "A history of modern init systems (1992-2015)"
init + runlevel configuration (integrated management)
|finit||Fast and extensible init, originally based on EeePC fastinit||2018-01-24|
|OpenRC|| Dependency-based parallel-process startup rc system that works with the system-provided init, normally SysVinit.
v0.25 onward includes openrc-init, which can replace /sbin/init (the default OpenRC provider for the init command is SysVinit)
|nosh||A suite of system-level utilities for initializing and running a BSD or Linux system (managing daemons / terminals / logging)||2017-12-11|
|anopa||An init system/service manager built around s6 supervision suite||2017-06-20|
|s6||an init + small suite of service supervision/management programs (ala daemontools and runit) for UNIX||2017-06-15|
|dinit||init system with process supervision, service dependencies and socket activation written in C++||2017-03-30|
|GNU PIES||Program Invocation and Execution Supervisor (inittab-compatible with a native rc format as well)||2016-10-01|
|procd||OpenWrt init and process management daemon with ubus integration||2018-02-05|
|Epoch||Single-threaded init system designed for minimal footprint, compatibility and unified configuration||2015-06-23|
|runit||UNIX init scheme with service supervision, a replacement for SysVinit, and other init schemes||2014-08-10|
|Upstart||Event-based init system which handles start/stop supervision of tasks and services (used in Chrome/Chromium OS)||2014-09-04|
|initng||Dependency-based init system with parallelization and asynchronous start||2007-11-11|
|eINIT||Can supervise processes and start them asynchronously, possibly without shell scripts||2007-12-18|
|minit||A very small init with process supervision, service dependency ordering and parallel service activation||2005 ?|
standalone init (separate from configuration/management of runlevels)
|SysV init||Traditional System V init..........Feb 2018: v 2.89b release notes / changelog||2018-02-27|
|busybox init||BusyBox init||2018-02-15|
|ueld||Similar to BSD init but more integrated||2017-06-24|
|uinit||Smallest init possible||2017-05-16|
|sninit||Small init implementation with SysV init like (sub)runlevels||2015-12-31|
|sinit||Simple init initially based on Rich Felker’s minimal init||2015-06-16|
|myinit||Simple init with parallel execution and dependencies via reference counting (`start' and `stop' commands)||2011-07-11|
|ninit||Fork from minit. See ninit Instructions||2010-01-16|
|simpleinit-msb||A fork of simpleinit (shipped with util-linux until v2.20); currently used in Source Mage GNU/Linux||2007-11-08|
service manager: a suite of programs which start and stop services, both long-running daemons and one-time initialization scripts, in the proper order according to a dependency tree
|s6-rc||service manager for s6-based systems||2018-01-04|
|Monit||process supervision tool for Unix and Linux; system status is viewable directly from commandline, or via a native HTTP(S) web server||2017-06-07|
|GNU Shepherd||service manager written in Guile scheme; used as PID 1 by GuixSD Linux||2016-12-04|
|watchman||not-so-simple service manager for Linux||2016-03-31|
|Supervisor||process control system written in Python||2016-02-06|
|freedt||a daemontools reimplementation||2014-09-03|
|daemontools encore||an enhanced version of daemontools github repository||2014-04-02|
|procer||process supervisor developed for mongrel2||2014-03-18|
|god (2)||an easily configurable, extensible, monitoring framework written in Ruby||2014-03-06|
|restartd||a process-restarting daemon (aka Debian restartd)||2013-01-11|
|perp||persistent process (service) supervisor and managment framework for UNIX||2013-01-11|
|daemontools||collection of tools for managing UNIX services||2001-07-12|
|startpar||can be used by the SysV RC boot system executor to allow parallel process system startup||2014-02-09|
|insserv||can be used with SysV -based init systems; provides dependency-driven system startup (dependencies are specified by LSB headers within init.d scripts)||2012-11-14|
- Gentoo wiki page: Comparison of init systems :Talk presents a more extensive / thorough "Detailed Feature Comparison Table"
- Gentoo wiki page: Comparison of init systems
- Manjaro wiki :: OpenRC
- Gentoo wiki :: OpenRC
- guide: Using OpenRC on Arch Linux
- VoidLinux wiki :: Runit
- Obarun wiki :: s6-boot
This without-systemd.org page is based on init article at ArchWiki. Content is available under GNU Free Documentation License 1.3 or later.